It's coming roughly a week after launch, but you now have a way to try Google Stadia without paying... provided you have the right connections. Google is rolling out its promised (but delayed) Buddy Passes to Stadia Founder's Edition buyers, giving them free access to Stadia Pro for three months. If you're a Stadia subscriber, you'll see an envelope icon in the mobile app that contains an invitation code you can send. If you're the recipient, you just have to redeem the code in the Stadia app to start playing.
There are caveats. The three-month freebie will convert to a $10 per month subscription once the Buddy Pass runs out, so you'll need to cancel service if you're unimpressed. And of course, Pro will only include four games as of December. If you're not sure you'll stick with Stadia in the long run, you'd better be happy with the likes of Destiny 2 and Tomb Raider. You should still have access to any purchased games once the free Stadia Base tier arrives, but it may be worth holding off on purchases until you know whether or not you can live with the basic service.
Still, this could be helpful in building the Stadia community. You may be more likely to stick with the service if you know a friend is playing, even if it's just for a few months. This also shows that Google is fleshing out Stadia's features, however slowly. There's still a long way to go (you won't find family sharing, for example), but it's an important start.